In erecting buildings, the surveyor is commonly charged with the task (among other tasks) of ensuring that all of the vertical structural elements are plumb and level, i.e., each vertical element (for example, a column, steel beam, or stud) must be installed so that it is precisely vertical with respect to gravity. In particular, each element must be plumb (vertical) in two directions. These two directions are sometimes referred to as “fore and aft” and “side-to-side” as viewed by a surveyor. In general, however, every structure has a perimeter, referred to herein as the structural envelope. In a simple rectangular plan structure, the envelope has four sides, but obviously there is no limit to the number of edges or even a prohibition on curved edges. Accordingly, the two directions may be generally understood to be “along the structural envelope” and “into the structural envelope,” respectively, in buildings of an arbitrary number of sides and curved vertical surfaces.
The typical surveying/erecting process for ensuring verticality or plumbing each structural element includes setting out an offset line parallel to each edge of the building perimeter at a known distance away from the actual building footprint. The offset distance is commonly from two to six feet from the centerline of the structural element, although surveyor's preference and site conditions usually dictate, as will be discussed below. A surveyor positioned at a known point on the offset line next takes a bearing (“shoots a line”) along the offset line and adjusts the vertical angle of this line to sight in a target attached to the structural element to be plumbed. The target may be any standard surveying target known to persons having ordinary skill in the art, such as a card target, retroreflector, or prism, and is typically attached (again by well-known means) to a target staff or builder's rod. The rod is typically held perpendicular to the vertical element's centerline and over the offset line by a helper known as the rod man. The target is positioned so that its center point or zero line is at the pre-determined offset distance from the building perimeter.
Since this process can take some time, and because it is unreasonable to expect the rod man to hold the rod perfectly still, the rod or staff is usually clamped to the column or other structural element. However, since the rod cannot be clamped to the column at its vertical center, the survey team must measure the distance from the column's centerline to the optical center of the target in order to ensure that all distance measurements are indexed relative to the vertical centerline of the structural element. These measurements must be made by hand to determine the compensation distances between the center of the column in both the fore-and-aft (along the structural envelope) and side-to-side (into the structural envelope) distances and the position of the target center. The compensation distances thus take into account the space between the target center and the true physical vertical centerline of the structural element. The rod must also be set perpendicular to the vertical center of the column, so that the compensation distance measurements are made on a straight line. This is time consuming and prone to both measurement and mathematical error.
Once the rod is set, the surveyor then directs the erectors to move the column into or out from the plane of the envelope (using conventional building steel erection methods) to plumb it in the lateral or side-to-side direction. Once plumbed in this first direction, the surveyor then measures the distance to the target along the offset line, using common surveying tools such as a theodolite equipped with electronic distance measurement (EDM) hardware and appropriate software. This process is well known in the art and will not be further described. Comparing this distance with the expected fore-and-aft position of the column, adjusted for the compensation distance between the rod's clamped position and the center of the column defined in the building plans, the surveyor then directs the erectors to adjust the column's position fore-and-aft, along the structural envelope.
The whole process is then repeated for the next structural element, and so on, down each edge of the structure and again on each successive floor. The offset distance may be varied to take into account structural or architectural projections (such as cantilevers) extending outside the building envelope or ground conditions (such as traffic) that preclude setting up the theodolite.
Prior art devices include the aforementioned builder's rods with permanent scales or graduations on them, which allow direct positioning to the offset line in the lateral direction. These devices are generally not furnished with clamping devices or other means of attaching them to a structural element. Also known is a magnetic-base target, such as the Sokkia RT50M, which can be directly attached to a steel column or other element. The RT50M, however, measures about four to eight inches from the magnetic base to the center of its retroreflective target and is not adjustable in length beyond a nominal amount. As such, it is too short to reach far enough outside of the building envelope to lie over a typical offset line. The Sokkia RT50M is made by Sokkia Co., Ltd. of 260-63 Hase, Atsugi, Kanagawa, Japan.
Another prior art device once used in the trade consists of a flat plate magnet with a shaft screwed perpendicular to the plane of the plate. Mounted on the shaft is a sliding target that can be positioned along its length. This device is prone to breakage as it can be easily knocked off the face of the column by passing workers, material, or equipment. It has no scale for direct measurement and requires careful setup to measure and compute the compensation distances from column center to target position.
A third prior art device is a tool known as an “erection rod.” This device consists of the long arm of a common carpenter's framing square (with its inscribed scale) mounted into a magnet that can be attached to a flat face (i.e., the flange) of a wide web column. A surveyor's target card is positioned on the arm by sliding it along the scale. This device, too, has problems with staying affixed to the steel and it also requires careful setup to measure and compute the compensation distances from column center to target position. Additionally, the target card is prone to blowing off in a stiff wind and movement in general. Furthermore, the whole device has to be rotated to place the target in view of the surveyor.
The deficiencies in the above-described conventional approaches include several sources of error and inaccuracy, potentially contributing to out-of-tolerance measurement accuracy. For example, the rod man must position the rod correctly on each structural element relative to its center and the offset line and measure—again, with high accuracy—the compensation distances from the column center in both the fore-and-aft and side-to-side directions. These offsets must be communicated to the surveyor without error and properly entered into the calculations. The target must also be pointed at the survey instrument, a direction that varies from column to column and floor to floor, which requires careful initial positioning of the entire tool.
What is needed is an adjustable alignment gauge that can be attached to a structural element and positioned rapidly, precisely, and repeatably with respect to the structural element's centerline and an offset line a short distance away from the building envelope.